The commonly used spice and flavoring agent, rosemary, derived from the leaves of the plant Rosmarinus officinalis L. has important antioxidant activity. Although carnosol, carnosic acid, rosmaridiphenol, rosmanol, isorosmanol, epirosmanol, and rosmariquinone are antioxidant compounds in rosemary leaves [Nakatani, N. in: Huang et al. (eds), Phenolic compounds in Food and Their Effects on Health II: Antioxidants and Cancer Prevention, pp. 72-86. Washington, DC: American Chemical Society, 1992] about 90% of the antioxidant activity of rosemary can be attributed to carnosol and carnosic acid [Aruoma et al. (1932) Xenobiotic 22:257-268]. We have demonstrated that other antioxidants such as the flavonol, quercetin, are potent stimulators of adriamycin (Adr) efflux via a P-glycoprotein (P-gp)-mediated mechanism [Critchfield et al. (1994) Biochem. Pharm. 48:1437-1445]. In this study, we examined the dietary herb, rosemary, which has both antioxidant and chemopreventive properties. Rosemary extract was partially purified and the antioxidant activity evaluated by the Ames test. The effect of rosemary was studied in human breast cancer drug-sensitive, wild-type (WT), R65, and MDR1 transfected (Clone 10.9) MCF-7 cells. Interestingly, we found that rosemary extract when present in 330- or 1000-fold excess competed for azidopine labelling of P-gp. We examined the rosemary effect on [14C] Adr efflux and found rosemary markedly decreased the P-gp mediated Adr efflux in R65 and Clone 10.9 but not WT MCF-7 cells. The cytotoxic effect of rosemary extract is extremely low as noted by the IC50 in WT and R65 cells of 125 mu M and 150 mu M, respectively. This is the first time that a natural product such as rosemary has been shown to inhibit P-gp mediated drug efflux. Our findings suggested that rosemary may serve as a MDR reversal agent for cancer chemotherapy.